1. Field of the Invention
The present invention relates to a magnetoresistance effect element and a magnetoresistance effect sensor used in a magnetic field sensor, a magnetic head, and the like and, more particularly, to a magnetoresistance effect element and a magnetoresistance effect sensor utilizing an artificial lattice film.
2. Description of the Related Art
The magnetoresistance effect is an effect in which the resistance of an object is changed upon application of a magnetic field. Magnetoresistance effect elements that utilize this effect find a variety of applications including those for magnetic field sensors and magnetic heads because of their high sensitivity to magnetic fields and their ability to produce a relatively large output. While a Permalloy thin film is conventionally widely used for such magnetoresistance effect elements, the magnetoresistance ratio (.DELTA.R/Rs: where .DELTA.R is a difference in electric resistance between the zero magnetic field and the saturated magnetic field; Rs is the electric resistance obtained when the saturation field is applied) of a Permalloy film is as low as about 2 to 3% and, therefore, does not show a satisfactory sensitivity to changes in the magnetic field.
On the other hand, as a new magnetoresistance effect element, a multilayer formed of alternately stacked magnetic and nonmagnetic layers each having a thickness of several to several tens of Angstroms or a so-called artificial lattice film has received a great deal of attention in recent years. Known types of the multilayers include (Fe/Cr).sub.n (Phys. Ref. Lett. vol. 61(21) (1988)2472, (Permalloy/Cu/Co/Cu).sub.n (J. Phys. Soc. Jap. vol. 59(9) (1990)3061), and (Co/Cu).sub.n (J. Mag. Mag. Mat. 94(1991)Ll; Phys. Rev. Lett. 66(1991)2152).
These multilayers exhibit a high magnetoresistance ratio as scores of %. In particular, when these multilayers are formed by using a film formation apparatus comprising an ultra-high vacuum system, e.g., an ultra-high vacuum (UHV) vapor deposition and a molecular beam epitaxy (MBE), a high magnetoresistance effect exceeding 20% can be obtained at room temperature. Accordingly, when these multilayers are used in the magnetoresistance effect heads, a large increase in output is expected.
However, a saturation field H.sub.S of the known multilayers are as strong as about 10 kOe in contrast to only several Oe for Permalloy. It follows that, where the known multilayers are used in a magnetic sensor or a magnetic head designed to detect a weak magnetic field, the magnetic sensor or head fails to exhibit a sufficient sensitivity.
More specifically, when an application as a magnetic sensor or a magnetic head is considered, it is desirable for the artificial lattice film to exhibit a large magnetoresistance change under a weak magnetic field. To this end, the saturation field H.sub.S of the artificial lattice film is required to be diminished.
However, an artificial lattice film meeting these particular requirements has not yet been developed.
It is proposed to use a magnetoresistance effect head utilizing the magnetoresistance effect described above to read data recorded on a magnetic recording medium (IEEE MAG-7, 150, (1971)). In recent years, as the size reduction and capacity increase of a magnetic recording medium have been in progress, the relative speed of the data reading magnetic head and the magnetic recording medium during data read has been decreased. Hence, a magnetoresistance effect head capable of obtaining a large output even at a low relative speed is expected.
When a magnetoresistance effect head is to be put into practical use, two types of bias magnetic fields must be applied to this head. One bias magnetic field is generally called a transverse bias to be applied in a direction perpendicular to the sense current of the magnetoresistance effect element. The transverse bias is a magnetic field which is applied until the magnitude of an external signal and that of a detection signal reach a proportional state, i.e., a so-called operating point. An example of a method of applying the transverse bias includes a self-bias scheme disclosed in Published Examined Japanese Patent Application Nos. 53-37205 and 56-40406 and the like and a shunt-bias scheme disclosed in Published Examined Japanese Patent Application No. 53-25646 and the like. According to the self-bias scheme, a soft adjacent layer is formed adjacent to a magnetoresistance effect film through a thin nonmagnetic film, and a magnetic field generated by the sense current is utilized as the transverse bias. A method of applying the transverse bias by flowing a current through a coil disposed adjacent to a magnetoresistance effect film is disclosed in Published Examined Japanese Patent Application No. 53-37206. A method of a hard magnetic film with a magnetization in one direction formed adjacent to a magnetoresistance effect film in order to apply the horizontal bias is disclosed in Published Examined Japanese Patent Application No. 54-8291 and the like.
The other bias magnetic field is generally called a longitudinal bias to be applied in a direction parallel to the sense current of the magnetoresistance effect element. The longitudinal bias suppresses Barkhausen noise which is caused since the magnetoresistance effect element has a large number of magnetic domains. In other words, the longitudinal bias serves to minimize the number of magnetic walls which cause noise generation.
Various methods have been conventionally proposed to apply the longitudinal bias. For example, U.S. Pat. No. 4,103,315 discloses that a uniform longitudinal bias is generated in a magnetoresistance effect film by exchange-coupling an antiferromagnetic film and a ferromagnetic film. According to JOURNAL OF APPLIED PHYSICS VOL. 52, 2472, (1981), when an FeMn alloy film is used as an antiferromagnetic film and a Permalloy (Ni.sub.80 Fe.sub.20) film is used as a magnetoresistance effect film, a vertical bias is applied to the magnetoresistance effect film due to the magnetic exchange coupling of the alloy and Permally films. In any of these cases, the spin of the magnetoresistance effect film is fixed in one direction by the longitudinal bias to suppress the Barkhausen noise.
As another example of the method of applying the longitudinal bias, in addition to the methods described above, a method of using an one-direction magnetized ferromagnetic film in the same manner as that employed for applying the transverse bias is proposed. According to this method, the longitudinal bias, the transverse bias, and an intermediate bias of the two biases can be applied by selecting the direction of magnetization. Magnetic Recording Laboratory, MR-37, the Institute of Electronic and Communication Engineers of Japan introduces a method of applying the longitudinal bias by forming a CoP film at the end portion of a yoke type magnetoresistance effect film.
In this manner, various methods have been proposed to apply the longitudinal bias. When, however, these methods are applied to the magnetic head for hard disk drive, the following problems arise.
Of these methods of applying the longitudinal bias, one with which the most preferable characteristics can be expected in an application to the magnetic head for hard disk drive is a method of forming an FeMn alloy (.gamma.-FeMn alloy) film as an antiferromagnetic film on a magnetoresistance effect film made of a Permalloy or the like.
The longitudinal bias magnetic field is desirably of 10 to 30 Oe.
However, the FeMn alloy seriously adversely affects the reliability of the magnetoresistance effect element, as is reported in Nippon Kinzoku Gakkai (Japanese Metal Society) (543), fall 1990, since Mn is easily oxidized. When an antiferromagnetic film is to be formed by sputtering a .gamma.-FeMn alloy, an .alpha.-FeMn alloy phase is sometimes formed, as is pointed out in JOURNAL OF APPLIED PHYSICS VOL. 52, 2471, (1981), and it is difficult to obtain a stable .gamma.-FeMn alloy phase in the industrial level.
Regarding the longitudinal bias to be applied, if it is weaker than the antimagnetic field at the edge portion of the magnetoresistance effect film, the magnetoresistance effect film fails to have a single magnetic domain; if it is weaker than this, the sensitivity of the magnetoresistance effect film is decreased. Hence, the longitudinal bias to be applied preferably has a strength to cancel the antimagnetic field at the edge portion of the magnetoresistance effect film. The strength of the antimagnetic field depends on the shape of the magnetoresistance effect element, i.e., the width and depth of the tracks, and the film thickness. Hence, the level of the exchange-coupling energy must be changed in accordance with the specifications of the magnetic head by controlling the shape of the magnetoresistance effect element. However, in order to control the exchange-coupling energy between the FeMn alloy as the antiferromagnetic film and the NiFe alloy as the magnetoresistance film, the thickness of the NiFe or FeMn alloy film must be changed, as described in the above JOURNAL OF APPLIED PHYSICS VOL. 52, 2471, (1981). When the thickness of the NiFe alloy film is changed, the characteristics of the magnetic head itself are changed. Thus, it is not preferable to arbitrarily change the thickness of the NiFe alloy film. When the thickness of the FeMn alloy film is increased, an .alpha.-FeMn alloy phase is formed in the film, which is similarly non-preferable. In this manner, it is actually very difficult to change the exchange-coupling energy in accordance with the specifications of the magnetic head.
Furthermore, as is pointed out in JOURNAL OF APPLIED PHYSICS VOL. 53, 2005, (1982), the exchange-coupling energy between the FeMn alloy and the NiFe alloy largely depends on the temperature, and the characteristics of the magnetoresistance effect element may be undesirably changed by the environmental conditions and heat generation by the sense current. In order to avoid these drawbacks, IEEE TRANS. MAG-24, 2609 (1988) discloses a method of exchange-coupling a TbCo alloy with an NiFe alloy. However, since the TbCo alloy is easily oxidized, long-term reliability is not guaranteed even if the environmental conditions where the alloy is to be used are limited.
The method of applying the longitudinal bias by the ferromagnetic body with a magnetization in one direction is effective if the magnetoresistance effect film is formed to be spaced apart from the magnetic recording medium, as in the yoke type magnetoresistance effect head. However, if the magnetoresistance effect film is formed close to the magnetic recording medium, as in the sealed type magnetoresistance effect head, the magnetic recording medium may undesirably be demagnetized by the leakage magnetic field from the ferromagnetic body. When the coercive force of the ferromagnetic member is decreased to avoid demagnetization, the direction of magnetization of the ferromagnetic body may undesirably be changed by the leakage magnetic field of the magnetic recording medium.
In this manner, the conventional methods of applying the longitudinal bias have various drawbacks when they are applied to a magnetic recording system in which a magnetoresistance effect film and a magnetic recording medium are adjacent to each other, as in a case wherein the magnetoresistance effect head for hard disk drive is used.